An up-to-date list is available on Google Scholar.

conferences & journals

2026

1. ICML

   Front-Loaded Robust Conformal Prediction: Heavy Calibration, Minimal Test-Time Cost

   Soroush H. Zargarbashi, Sadegh Mohammad Akhondzadeh, and Aleksandar Bojchevski

   In International Conference on Machine Learning, ICML, 2026

   Abs HTML PDF

   Robust conformal prediction (RCP) extends conformal prediction (CP) to noisy inputs by producing prediction sets with guaranteed coverage, ensuring that the true label is contained in the set with a user-specified probability even under worst-case perturbations. Recent works use randomized smoothing, as it provides robustness for black-box models at larger radii. Currently, there exist two setups for smoothing-based RCP: one requires extensive Monte Carlo sampling at calibration and test time but results in smaller prediction sets; the other setup produces larger prediction sets but uses a single sample at both stages. In deployment, calibration—as a one-time pre-processing step—can accommodate substantially higher computational overhead than inference. Inspired by this observation, we introduce an RCP framework that strikes a balance between the two extremes of this trade-off: we increase the sample rate at calibration time while keeping it either one or very low during test time. This calibration-time sampling opens the possibility of reducing the size of the prediction sets. In production, where the number of test predictions typically far exceeds the size of the calibration set, our Front-Loaded RCP matches the computational complexity of the state of the art while producing considerably smaller prediction sets at larger radii.

2. ICLR

   EvA: Evolutionary Attacks on Graphs

   Sadegh Mohammad Akhondzadeh, Soroush H. Zargarbashi, Jimin Cao, and Aleksandar Bojchevski

   In International Conference on Learning Representations, ICLR, 2026

   Abs arXiv

   Even a slight perturbation in the graph structure can cause a significant drop in the accuracy of graph neural networks (GNNs). Most existing attacks leverage gradient information to perturb edges. This relaxes the attack’s optimization problem from a discrete to a continuous space, resulting in solutions far from optimal. It also restricts the adaptability of the attack to non-differentiable objectives. Instead, we introduce a few simple yet effective enhancements of an evolutionary-based algorithm to solve the discrete optimization problem directly. Our Evolutionary Attack (EvA) works with any black-box model and objective, eliminating the need for a differentiable proxy loss. This allows us to design two novel attacks that reduce the effectiveness of robustness certificates and break conformal sets. The memory complexity of our attack is linear in the attack budget. Among our experiments, EvA shows \(∼\)11% additional drop in accuracy on average compared to the best previous attack, revealing significant untapped potential in designing attacks.

3. UAI

   Optimal Conformal Prediction under Epistemic Uncertainty

   Alireza Javanmardi, Soroush H. Zargarbashi, Santo M. A. R. Thies, Willem Waegeman, Aleksandar Bojchevski, and Eyke Hüllermeier

   In Conference on Uncertainty in Artificial Intelligence, UAI, 2026

   Abs arXiv

   Conformal prediction (CP) is a widely used frequentist framework to quantify uncertainty by constructing prediction sets with user-specified marginal coverage guarantees. In practice, CP is typically applied on top of probabilistic classifiers, which are able to express aleatoric but not epistemic uncertainty. In this paper, we consider the question of how to optimally employ CP on top of a more expressive formalism, namely credal sets, which can express both aleatoric and epistemic uncertainty. More specifically, we propose probabilistic Bernoulli prediction sets (BPS) and derive a variant that achieves conditional coverage for valid credal sets while remaining minimal in expected size. We then address the more realistic scenario in which the validity of the credal sets is not guaranteed. Assuming access to calibration data with ground-truth distributions over labels, we apply conformal risk control to BPS and derive a PAC-style guarantee: with high probability over the data, the achieved conditional coverage is at least the desired level. We validate our theoretical findings empirically over various datasets.

4. EnergyAI

   SafePowerGraph: A Safety-Aware Benchmarking Framework for Scalable Graph Neural Networks in Power Flow and Optimal Power Flow

   Aoxiang Ma, Salah Ghamizi, Aleksandar Bojchevski, Pei Zhang, and Jun Cao

   Energy and AI, 2026

   Abs arXiv

   Power grids are critical infrastructures of paramount importance to modern society and their rapid evolution and interconnections has heightened the complexity of power systems (PS) operations. Traditional methods for grid analysis struggle with the computational demands of large-scale RES and ES integration, prompting the adoption of machine learning (ML) techniques, particularly Graph Neural Networks (GNNs). GNNs have proven effective in solving the alternating current (AC) Power Flow (PF) and Optimal Power Flow (OPF) problems, crucial for operational planning. However, existing benchmarks and datasets completely ignore safety and robustness requirements in their evaluation and never consider realistic safety-critical scenarios that most impact the operations of the power grids. We present SafePowerGraph, the first simulator-agnostic, safety-oriented framework and benchmark for GNNs in PS operations. SafePowerGraph integrates multiple PF and OPF simulators and assesses GNN performance under diverse scenarios, including energy price variations and power line outages. Our extensive experiments underscore the importance of self-supervised learning and graph attention architectures for GNN robustness. We provide at https://github.com/yamizi/SafePowerGraph our open-source repository, a comprehensive leaderboard, a dataset and model zoo and expect our framework to standardize and advance research in the critical field of GNN for power systems.

2025

1. NeurIPS

   One Sample is Enough to Make Conformal Prediction Robust

   Soroush H. Zargarbashi, Sadegh Mohammad Akhondzadeh, and Aleksandar Bojchevski

   In Neural Information Processing Systems, NeurIPS, 2025

   Abs arXiv

   For any black-box model, conformal prediction (CP) returns prediction sets guaranteed to include the true label with high adjustable probability. Robust CP (RCP) extends the guarantee to the worst case noise up to a pre-defined magnitude. For RCP, a well-established approach is to use randomized smoothing since it is applicable to any black-box model and provides smaller sets compared to deterministic methods. However, smoothing-based robustness requires many model forward passes per each input which is computationally expensive. We show that conformal prediction attains some robustness even with a single forward pass on a randomly perturbed input. Using any binary certificate we propose a single sample robust CP (RCP1). Our approach returns robust sets with smaller average set size compared to SOTA methods which use many (e.g. 100) passes per input. Our key insight is to certify the conformal procedure itself rather than individual conformity scores. Our approach is agnostic to the task (classification and regression). We further extend our approach to smoothing-based robust conformal risk control.

2. EMNLP

   KurTail: Kurtosis-Based LLM Quantization

   Sadegh Mohammad Akhondzadeh, Aleksandar Bojchevski, Evangelos Eleftheriou, and Martino Dazzi

   In Findings of the Association for Computational Linguistics, EMNLP, 2025

   Abs arXiv

   One of the challenges of quantizing a large language model (LLM) is the presence of outliers. Outliers often make uniform quantization schemes less effective, particularly in extreme cases such as 4-bit quantization. We introduce KurTail, a new post-training quantization (PTQ) scheme that leverages Kurtosis-based rotation to mitigate outliers in the activations of LLMs. Our method optimizes Kurtosis as a measure of tailedness. This approach enables the quantization of weights, activations, and the KV cache in 4 bits. We utilize layer-wise optimization, ensuring memory efficiency. KurTail outperforms existing quantization methods, offering a 13.3% boost in MMLU accuracy and a 15.5% drop in Wiki perplexity compared to QuaRot. It also outperforms SpinQuant with a 2.6% MMLU gain and reduces perplexity by 2.9%, all while reducing the training cost. For comparison, learning the rotation using SpinQuant for Llama3-70B requires at least four NVIDIA H100 80GB GPUs, whereas our method requires only a single GPU, making it a more accessible solution for consumer GPU.

3. ICLR

   Robust Conformal Prediction with a Single Binary Certificate

   Soroush H. Zargarbashi and Aleksandar Bojchevski

   In International Conference on Learning Representations, ICLR, 2025

   Abs arXiv Slides

   Conformal prediction (CP) converts any model’s output to prediction sets with a guarantee to cover the true label with (adjustable) high probability. Robust CP extends this guarantee to worst-case (adversarial) inputs. Existing baselines achieve robustness by bounding randomly smoothed conformity scores. In practice, they need expensive Monte-Carlo (MC) sampling (e.g. \(\sim10^4\)samples per point) to maintain an acceptable set size. We propose a robust conformal prediction that produces smaller sets even with significantly lower MC samples (e.g. 150 for CIFAR10). Our approach binarizes samples with an adjustable (or automatically adjusted) threshold selected to preserve the coverage guarantee. Remarkably, we prove that robustness can be achieved by computing only one binary certificate, unlike previous methods that certify each calibration (or test) point. Thus, our method is faster and returns smaller robust sets. We also eliminate a previous limitation that requires a bounded score function.

4. TMLR

   Node-Level Data Valuation on Graphs

   Simone Antonelli and Aleksandar Bojchevski

   Transactions on Machine Learning Research, TMLR, 2025

   Abs HTML PDF Code

   How much is a node worth? We answer this question using an emerging set of data valuation techniques, where the value of a data point is measured via its marginal contribution when added to the (training) dataset. Data valuation has been primarily studied in the i.i.d. setting, giving rise to methods like influence functions, leave-one-out estimation, data Shapley, and data Banzhaf. We conduct a comprehensive study of data valuation approaches applied to graph-structured models such as graph neural networks in a semi-supervised transductive setting. Since all nodes (labeled and unlabeled) influence both training and inference we construct various scenarios to understand the diverse mechanisms by which nodes can impact learning. We show that the resulting node values can be used to identify (positively and negatively) influential nodes, quantify model brittleness, detect poisoned data, and accurately predict counterfactuals.

5. KAIS

   Evaluating the Transferability of Adversarial Robustness to Target Domains

   Anna-Kathrin Kopetzki, Aleksandar Bojchevski, and Stephan Günnemann

   Knowledge and Information Systems, 2025

   Abs DOI

   Knowledge transfer is an effective method for learning, particularly useful when labeled data are limited or when training a model from scratch is too expensive. Most of the research on transfer learning focuses on achieving accurate models, overlooking the crucial aspect of adversarial robustness. However, ensuring robustness is vital, especially when applying transfer learning in safety-critical domains. We compare robustness of models obtained by 11 training procedures on source domains and 3 retraining schemes on target domains, including normal, adversarial, contrastive, and Lipschitz constrained training variants. Robustness is analyzed by adversarial attacks with respect to two different transfer learning model outputs: (i) the latent representations and (ii) the predictions. Studying latent representations in correlation with predictions is crucial for robustness of transfer learning models, since they are solely learned on the source domain. Besides adversarial attacks that aim at changing the prediction, we also analyze the effect of directly attacking representations. Our results show that adversarial robustness can transfer across domains, but effective robust transfer learning requires techniques that ensure robustness independent of the training data to preserve them during the transfer. Retraining on the target domain has a minor impact on the robustness of the target model. Representations exhibit greater robustness compared to predictions across both the source and target domain.

2024

1. NeurIPS

   SVFT: Parameter-Efficient Fine-Tuning with Singular Vectors

   Vijay Lingam, Atula Tejaswi, Aditya Vavre, Aneesh Shetty, Gautham Krishna Gudur, Joydeep Ghosh, Alex Dimakis, Eunsol Choi, Aleksandar Bojchevski, and Sujay Sanghavi

   In Neural Information Processing Systems, NeurIPS, 2024

   Abs arXiv HTML PDF Code

   Popular parameter-efficient fine-tuning (PEFT) methods, such as LoRA and its variants, freeze pre-trained model weights \(W\)and inject learnable matrices \(∆W\). These \(∆W\)matrices are structured for efficient parameterization, often using techniques like low-rank approximations or scaling vectors. However, these methods typically show a performance gap compared to full fine-tuning. Although recent PEFT methods have narrowed this gap, they do so at the cost of additional learnable parameters. We propose SVFT, a simple approach that fundamentally differs from existing methods: the structure imposed on \(∆W\)depends on the specific weight matrix \(W\). Specifically, SVFT updates \(W\)as a sparse combination of outer products of its singular vectors, training only the coefficients (scales) of these sparse combinations. This approach allows fine-grained control over expressivity through the number of coefficients. Extensive experiments on language and vision benchmarks show that SVFT recovers up to 96% of full fine-tuning performance while training only 0.006 to 0.25% of parameters, outperforming existing methods that only recover up to 85% performance using 0.03 to 0.8% of the trainable parameter budget.

2. ICML

   Robust Yet Efficient Conformal Prediction Sets

   Soroush H. Zargarbashi, Sadegh Mohammad Akhondzadeh, and Aleksandar Bojchevski

   In International Conference on Machine Learning, ICML, 2024

   Abs arXiv HTML PDF Code Poster

   Conformal prediction (CP) can convert any model’s output into prediction sets guaranteed to include the true label with any user-specified probability. However, same as the model itself, CP is vulnerable to adversarial test examples (evasion) and perturbed calibration data (poisoning). We derive provably robust sets by bounding the worst-case change in conformity scores. Our tighter bounds lead to more efficient sets. We cover both continuous and discrete (sparse) data and our guarantees work both for evasion and poisoning attacks (on both features and labels).

3. ICLR

   Conformal Inductive Graph Neural Networks

   Soroush H. Zargarbashi and Aleksandar Bojchevski

   In International Conference on Learning Representations, ICLR, 2024

   Abs arXiv HTML PDF Code Poster

   Conformal prediction (CP) transforms any model’s output into prediction sets guaranteed to include (cover) the true label. CP requires exchangeability, a relaxation of the i.i.d. assumption, to obtain a valid distribution-free coverage guarantee. This makes it directly applicable to transductive node-classification. However, conventional CP cannot be applied in inductive settings due to the implicit shift in the (calibration) scores caused by message passing with the new nodes. We fix this issue for both cases of node and edge-exchangeable graphs, recovering the standard coverage guarantee without sacrificing statistical efficiency. We further prove that the guarantee holds independently of the prediction time, e.g. upon arrival of a new node/edge or at any subsequent moment.

4. ICLR

   Rethinking Label Poisoning for GNNs: Pitfalls and Attacks

   Vijay Lingam, Sadegh Mohammad Akhondzadeh, and Aleksandar Bojchevski

   In International Conference on Learning Representations, ICLR, 2024

   Abs HTML PDF Code Poster

   Node labels for graphs are usually generated using an automated process or crowd-sourced from human users. This opens up avenues for malicious users to compromise the training labels, making it unwise to blindly rely on them. While robustness against noisy labels is an active area of research, there are only a handful of papers in the literature that address this for graph-based data. Even more so, the effects of adversarial label perturbations is sparsely studied. More critically, we reveal that the entire literature on label poisoning for GNNs is plagued by serious evaluation pitfalls. Thus making it hard to conclude how robust GNNs are against label perturbations. After course correcting the state of label poisoning attacks with our faithful evaluation, we identify a discrepancy in attack efficiency of 9% on average. Additionally, we introduce two new simple yet effective attacks that are significantly stronger (up to 8%) than the previous strongest attack. Our strongest proposed attack can be efficiently computed and is theoretically backed.

2023

1. NeurIPS

   Hierarchical Randomized Smoothing

   Yan Scholten, Jan Schuchardt, Aleksandar Bojchevski, and Stephan Günnemann

   In Neural Information Processing Systems, NeurIPS, 2023

   Abs arXiv PDF Video Code Poster

   Real-world data is complex and often consists of objects that can be decomposed into multiple entities (e.g. images into pixels, graphs into interconnected nodes). Randomized smoothing is a powerful framework for making models provably robust against small changes to their inputs - by guaranteeing robustness of the majority vote when randomly adding noise before classification. Yet, certifying robustness on such complex data via randomized smoothing is challenging when adversaries do not arbitrarily perturb entire objects (e.g. images) but only a subset of their entities (e.g. pixels). As a solution, we introduce hierarchical randomized smoothing: We partially smooth objects by adding random noise only on a randomly selected subset of their entities. By adding noise in a more targeted manner than existing methods we obtain stronger robustness guarantees while maintaining high accuracy. We initialize hierarchical smoothing using different noising distributions, yielding novel robustness certificates for discrete and continuous domains. We experimentally demonstrate the importance of hierarchical smoothing in image and node classification, where it yields superior robustness-accuracy trade-offs. Overall, hierarchical smoothing is an important contribution towards models that are both - certifiably robust to perturbations and accurate.

2. NeurIPS

   Are GATs Out of Balance?

   Nimrah Mustafa, Aleksandar Bojchevski, and Rebekka Burkholz

   In Neural Information Processing Systems, NeurIPS, 2023

   Abs arXiv Code Poster

   While the expressive power and computational capabilities of graph neural networks (GNNs) have been theoretically studied, their optimization and learning dynamics, in general, remain largely unexplored. Our study undertakes the Graph Attention Network (GAT), a popular GNN architecture in which a node’s neighborhood aggregation is weighted by parameterized attention coefficients. We derive a conservation law of GAT gradient flow dynamics, which explains why a high portion of parameters in GATs with standard initialization struggle to change during training. This effect is amplified in deeper GATs, which perform significantly worse than their shallow counterparts. To alleviate this problem, we devise an initialization scheme that balances the GAT network. Our approach i) allows more effective propagation of gradients and in turn enables trainability of deeper networks, and ii) attains a considerable speedup in training and convergence time in comparison to the standard initialization. Our main theorem serves as a stepping stone to studying the learning dynamics of positive homogeneous models with attention mechanisms.

3. ICML

   Conformal Prediction Sets for Graph Neural Networks

   Soroush H. Zargarbashi, Simone Antonelli, and Aleksandar Bojchevski

   In International Conference on Machine Learning, ICML, 2023

   Abs HTML PDF Code Poster

   Despite the widespread use of graph neural networks (GNNs) we lack methods to reliably quantify their uncertainty. We propose a conformal procedure to equip GNNs with prediction sets that come with distribution-free guarantees – the output set contains the true label with arbitrarily high probability. Our post-processing procedure can wrap around any (pretrained) GNN, and unlike existing methods, results in meaningful sets even when the model provides only the top class. The key idea is to diffuse the node-wise conformity scores to incorporate neighborhood information. By leveraging the network homophily we construct sets with comparable or better efficiency (average size) and significantly improved singleton hit ratio (correct sets of size one). In addition to an extensive empirical evaluation, we investigate the theoretical conditions under which smoothing provably improves efficiency.

4. AISTATS

   Probing Graph Representations

   Sadegh Mohammad Akhondzadeh, Vijay Lingam, and Aleksandar Bojchevski

   In International Conference on Artificial Intelligence and Statistics, AISTATS, 2023

   Abs arXiv Code

   Today we have a good theoretical understanding of the representational power of Graph Neural Networks (GNNs). For example, their limitations have been characterized in relation to a hierarchy of Weisfeiler-Lehman (WL) isomorphism tests. However, we do not know what is encoded in the learned representations. This is our main question. We answer it using a probing framework to quantify the amount of meaningful information captured in graph representations. Our findings on molecular datasets show the potential of probing for understanding the inductive biases of graph-based models. We compare different families of models, and show that Graph Transformers capture more chemically relevant information compared to models based on message passing. We also study the effect of different design choices such as skip connections and virtual nodes. We advocate for probing as a useful diagnostic tool for evaluating and developing graph-based models.

5. ICLR

   Unveiling the Sampling Density in Non-uniform Geometric Graphs

   Raffaele Paolino, Aleksandar Bojchevski, Stephan Günnemann, Gitta Kutyniok, and Ron Levie

   In International Conference on Learning Representation, ICLR, 2023

   Abs arXiv

   A powerful framework for studying graphs is to consider them as geometric graphs: nodes are randomly sampled from an underlying metric space, and any pair of nodes is connected if their distance is less than a specified neighborhood radius. Currently, the literature mostly focuses on uniform sampling and constant neighborhood radius. However, real-world graphs are likely to be better represented by a model in which the sampling density and the neighborhood radius can both vary over the latent space. For instance, in a social network communities can be modeled as densely sampled areas, and hubs as nodes with larger neighborhood radius. In this work, we first perform a rigorous mathematical analysis of this (more general) class of models, including derivations of the resulting graph shift operators. The key insight is that graph shift operators should be corrected in order to avoid potential distortions introduced by the non-uniform sampling. Then, we develop methods to estimate the unknown sampling density in a self-supervised fashion. Finally, we present exemplary applications in which the learnt density is used to 1) correct the graph shift operator and improve performance on a variety of tasks, 2) improve pooling, and 3) extract knowledge from networks. Our experimental findings support our theory and provide strong evidence for our model.

6. ICLR Notable

   Localized Randomized Smoothing for Collective Robustness Certification

   Jan Schuchardt, Tom Wollschläger, Aleksandar Bojchevski, and Stephan Günnemann

   In International Conference on Learning Representation, ICLR, 2023

   Abs arXiv

   Models for image segmentation, node classification and many other tasks map a single input to multiple labels. By perturbing this single shared input (e.g. the image) an adversary can manipulate several predictions (e.g. misclassify several pixels). Collective robustness certification is the task of provably bounding the number of robust predictions under this threat model. The only dedicated method that goes beyond certifying each output independently is limited to strictly local models, where each prediction is associated with a small receptive field. We propose a more general collective robustness certificate for all types of models and further show that this approach is beneficial for the larger class of softly local models, where each output is dependent on the entire input but assigns different levels of importance to different input regions (e.g. based on their proximity in the image). The certificate is based on our novel localized randomized smoothing approach, where the random perturbation strength for different input regions is proportional to their importance for the outputs. Localized smoothing Pareto-dominates existing certificates on both image segmentation and node classification tasks, simultaneously offering higher accuracy and stronger guarantees.

7. AAAI Oral

   Adversarial Weight Perturbation Improves Generalization in Graph Neural Networks

   Yihan Wu, Aleksandar Bojchevski, and Heng Huang

   In Conference on Artificial Intelligence, AAAI, 2023

   Abs arXiv Code

   A lot of theoretical and empirical evidence shows that the flatter local minima tend to improve generalization. Adversarial Weight Perturbation (AWP) is an emerging technique to efficiently and effectively find such minima. In AMP we minimize the loss w.r.t. a bounded worst-case perturbation of the model parameters thereby favoring local minima with a small loss in a neighborhood around them. The benefits of AWP, and more generally the connections between flatness and generalization, have been extensively studied for i.i.d. data such as images. In this paper, we extensively study this phenomenon for graph data. Along the way, we first derive a generalization bound for non-i.i.d. node classification tasks. Then we identify a vanishing-gradient issue with all existing formulations of AWP and we propose a new Weighted Truncated AWP (WT-AWP) to alleviate this issue. We show that regularizing graph neural networks with WT-AWP consistently improves both natural and robust generalization across many different graph learning tasks and models.

2022

1. NeurIPS

   Are Defenses for Graph Neural Networks Robust?

   Felix Mujkanovic, Simon Geisler, Stephan Günnemann, and Aleksandar Bojchevski

   In Neural Information Processing Systems, NeurIPS, 2022

   Abs arXiv Video Code

   A cursory reading of the literature suggests that we made a lot of progress in designing effective adversarial defenses for Graph Neural Networks (GNNs). Yet, the standard methodology has a serious flaw – virtually all of the defenses are evaluated against non-adaptive attacks leading to overly optimistic robustness estimates. We perform a thorough robustness analysis of 7 of the most popular defenses spanning the entire spectrum of strategies, i.e. aimed at improving the graph, the architecture, or the training. The results are sobering – most defenses show no or only marginal improvement compared to an undefended baseline. We advocate using custom adaptive attacks as a gold standard and we outline the lessons we learned from successfully designing such attacks. Moreover, our diverse collection of perturbed graphs forms a (black-box) unit test offering a first glance at a model’s robustness.

2. NeurIPS

   Randomized Message-Interception Smoothing: Gray-box Certificates for Graph Neural Networks

   Yan Scholten, Jan Schuchardt, Simon Geisler, Aleksandar Bojchevski, and Stephan Günnemann

   In Neural Information Processing Systems, NeurIPS, 2022

   Abs arXiv Video Code Poster

   Randomized smoothing is one of the most promising frameworks for certifying the adversarial robustness of machine learning models, including Graph Neural Networks (GNNs). Yet, existing randomized smoothing certificates for GNNs are overly pessimistic since they treat the model as a black box, ignoring the underlying architecture. To remedy this, we propose novel gray-box certificates that exploit the message-passing principle of GNNs: We randomly intercept messages and carefully analyze the probability that messages from adversarially controlled nodes reach their target nodes. Compared to existing certificates, we certify robustness to much stronger adversaries that control entire nodes in the graph and can arbitrarily manipulate node features. Our certificates provide stronger guarantees for attacks at larger distances, as messages from farther-away nodes are more likely to get intercepted. We demonstrate the effectiveness of our method on various models and datasets. Since our gray-box certificates consider the underlying graph structure, we can significantly improve certifiable robustness by applying graph sparsification.

3. ICLR

   Generalization of Neural Combinatorial Solvers Through the Lens of Adversarial Robustness

   Simon Geisler, Johanna Sommer, Jan Schuchardt, Aleksandar Bojchevski, and Stephan Günnemann

   In International Conference on Learning Representation, ICLR, 2022

   Abs HTML PDF Video

   End-to-end (geometric) deep learning has seen first successes in approximating the solution of combinatorial optimization problems. However, generating data in the realm of NP-hard/-complete tasks brings practical and theoretical challenges, resulting in evaluation protocols that are too optimistic. Specifically, most datasets only capture a simpler subproblem and likely suffer from spurious features. We investigate these effects by studying adversarial robustness – a local generalization property – to reveal hard, model-specific instances and spurious features. For this purpose, we derive perturbation models for SAT and TSP. Unlike in other applications, where perturbation models are designed around subjective notions of imperceptibility, our perturbation models are efficient and sound, allowing us to determine the true label of perturbed samples without a solver. Surprisingly, with such perturbations, a sufficiently expressive neural solver does not suffer from the limitations of the accuracy-robustness trade-off common in supervised learning. Although such robust solvers exist, we show empirically that the assessed neural solvers do not generalize well w.r.t. small perturbations of the problem instance.

2021

1. NeurIPS

   Robustness of Graph Neural Networks at Scale

   Simon Geisler, Thomas Schmidt, Hakan Şirin, Daniel Zügner, Aleksandar Bojchevski, and Stephan Günnemann

   In Neural Information Processing Systems, NeurIPS, 2021

   Abs arXiv Video Code

   Graph Neural Networks (GNNs) are increasingly important given their popularity and the diversity of applications. Yet, existing studies of their vulnerability to adversarial attacks rely on relatively small graphs. We address this gap and study how to attack and defend GNNs at scale. We propose two sparsity-aware first-order optimization attacks that maintain an efficient representation despite optimizing over a number of parameters which is quadratic in the number of nodes. We show that common surrogate losses are not well-suited for global attacks on GNNs. Our alternatives can double the attack strength. Moreover, to improve GNNs’ reliability we design a robust aggregation function, Soft Median, resulting in an effective defense at all scales. We evaluate our attacks and defense with standard GNNs on graphs more than 100 times larger compared to previous work. We even scale one order of magnitude further by extending our techniques to a scalable GNN.

2. ICLR

   Collective Robustness Certificates: Exploiting Interdependence in Graph Neural Networks

   Jan Schuchardt, Aleksandar Bojchevski, Johannes Gasteiger, and Stephan Günnemann

   In International Conference on Learning Representations, ICLR, 2021

   Abs HTML PDF Video Code Poster Slides

   In tasks like node classification, image segmentation, and named-entity recognition we have a classifier that simultaneously outputs multiple predictions (a vector of labels) based on a single input, i.e. a single graph, image, or document respectively. Existing adversarial robustness certificates consider each prediction independently and are thus overly pessimistic for such tasks. They implicitly assume that an adversary can use different perturbed inputs to attack different predictions, ignoring the fact that we have a single shared input. We propose the first collective robustness certificate which computes the number of predictions that are simultaneously guaranteed to remain stable under perturbation, i.e. cannot be attacked. We focus on Graph Neural Networks and leverage their locality property - perturbations only affect the predictions in a close neighborhood - to fuse multiple single-node certificates into a drastically stronger collective certificate. For example, on the Citeseer dataset our collective certificate for node classification increases the average number of certifiable feature perturbations from 7 to 351.

3. AISTATS

   Completing the Picture: Randomized Smoothing Suffers from the Curse of Dimensionality for a Large Family of Distributions

   Yihan Wu, Aleksandar Bojchevski, Aleksei Kuvshinov, and Stephan Günnemann

   In International Conference on Artificial Intelligence and Statistics, AISTATS, 2021

   Abs PDF Video Code Poster

   Randomized smoothing is currently the most competitive technique for providing provable robustness guarantees. Since this approach is model-agnostic and inherently scalable we can certify arbitrary classifiers. Despite its success, recent works show that for a small class of i.i.d. distributions, the largest lp radius that can be certified using randomized smoothing decreases as O(1/d^(1/2-1/p)) with dimension d for p > 2. We complete the picture and show that similar no-go results hold for the l2 norm for a much more general family of distributions which are continuous and symmetric about the origin. Specifically, we calculate two different upper bounds of the l2 certified radius which have a constant multiplier of order Theta(1/d^1/2). Moreover, we extend our results to lp (p>2) certification with spherical symmetric distributions solidifying the limitations of randomized smoothing. We discuss the implications of our results for how accuracy and robustness are related, and why robust training with noise augmentation can alleviate some of the limitations in practice. We also show that on real-world data the gap between the certified radius and our upper bounds is small.

2020

1. ICML

   Efficient Robustness Certificates for Discrete Data: Sparsity-Aware Randomized Smoothing for Graphs, Images and More

   Aleksandar Bojchevski, Johannes Gasteiger, and Stephan Günnemann

   In International Conference on Machine Learning, ICML, 2020

   Abs arXiv Video Code Slides

   Existing techniques for certifying the robustness of models for discrete data either work only for a small class of models or are general at the expense of efficiency or tightness. Moreover, they do not account for sparsity in the input which, as our findings show, is often essential for obtaining non-trivial guarantees. We propose a model-agnostic certificate based on the randomized smoothing framework which subsumes earlier work and is tight, efficient, and sparsity-aware. Its computational complexity does not depend on the number of discrete categories or the dimension of the input (e.g. the graph size), making it highly scalable. We show the effectiveness of our approach on a wide variety of models, datasets, and tasks – specifically highlighting its use for Graph Neural Networks. So far, obtaining provable guarantees for GNNs has been difficult due to the discrete and non-i.i.d. nature of graph data. Our method can certify any GNN and handles perturbations to both the graph structure and the node attributes.

2. KDD Oral

   Scaling Graph Neural Networks with Approximate PageRank

   Aleksandar Bojchevski^*, Johannes Gasteiger^*, Bryan Perozzi, Amol Kapoor, Martin Blais, Benedek Rózemberczki, Michal Lukasik, and Stephan Günnemann

   In International Conference on Knowledge Discovery and Data Mining, KDD, 2020

   Abs arXiv Video Code Slides

   Graph neural networks (GNNs) have emerged as a powerful approach for solving many network mining tasks. However, learning on large graphs remains a challenge - many recently proposed scalable GNN approaches rely on an expensive message-passing procedure to propagate information through the graph. We present the PPRGo model which utilizes an efficient approximation of information diffusion in GNNs resulting in significant speed gains while maintaining state-of-the-art prediction performance. In addition to being faster, PPRGo is inherently scalable, and can be trivially parallelized for large datasets like those found in industry settings. We demonstrate that PPRGo outperforms baselines in both distributed and single-machine training environments on a number of commonly used academic graphs. To better analyze the scalability of large-scale graph learning methods, we introduce a novel benchmark graph with 12.4 million nodes, 173 million edges, and 2.8 million node features. We show that training PPRGo from scratch and predicting labels for all nodes in this graph takes under 2 minutes on a single machine, far outpacing other baselines on the same graph. We discuss the practical application of PPRGo to solve large-scale node classification problems at Google.

3. ALENEX

   Group Centrality Maximization for Large-scale Graphs

   Eugenio Angriman, Alexander Grinten, Aleksandar Bojchevski, Daniel Zügner, Stephan Günnemann, and Henning Meyerhenke

   In Symposium on Algorithm Engineering and Experiments, ALENEX, 2020

   Abs arXiv Code

   The study of vertex centrality measures is a key aspect of network analysis. Naturally, such centrality measures have been generalized to groups of vertices; for popular measures it was shown that the problem of finding the most central group is NP-hard. As a result, approximation algorithms to maximize group centralities were introduced recently. Despite a nearly-linear running time, approximation algorithms for group betweenness and (to a lesser extent) group closeness are rather slow on large networks due to high constant overheads. That is why we introduce GED-Walk centrality, a new submodular group centrality measure inspired by Katz centrality. In contrast to closeness and betweenness, it considers walks of any length rather than shortest paths, with shorter walks having a higher contribution. We define algorithms that (i) efficiently approximate the GED-Walk score of a given group and (ii) efficiently approximate the (proved to be NP-hard) problem of finding a group with highest GED-Walk score. Experiments on several real-world datasets show that scores obtained by GED-Walk improve performance on common graph mining tasks such as collective classification and graph-level classification. An evaluation of empirical running times demonstrates that maximizing GED-Walk (in approximation) is two orders of magnitude faster compared to group betweenness approximation and for group sizes ≤100 one to two orders faster than group closeness approximation. For graphs with tens of millions of edges, approximate GED-Walk maximization typically needs less than one minute. Furthermore, our experiments suggest that the maximization algorithms scale linearly with the size of the input graph and the size of the group.

2019

1. NeurIPS

   Certifiable Robustness to Graph Perturbations

   Aleksandar Bojchevski and Stephan Günnemann

   In Neural Information Processing Systems, NeurIPS, 2019

   Abs arXiv Code Poster

   Despite the exploding interest in graph neural networks there has been little effort to verify and improve their robustness. This is even more alarming given recent findings showing that they are extremely vulnerable to adversarial attacks on both the graph structure and the node attributes. We propose the first method for verifying certifiable (non-)robustness to graph perturbations for a general class of models that includes graph neural networks and label/feature propagation. By exploiting connections to PageRank and Markov decision processes our certificates can be efficiently (and under many threat models exactly) computed. Furthermore, we investigate robust training procedures that increase the number of certifiably robust nodes while maintaining or improving the clean predictive accuracy.

2. ICML Oral

   Adversarial Attacks on Node Embeddings via Graph Poisoning

   Aleksandar Bojchevski and Stephan Günnemann

   In International Conference on Machine Learning, ICML, 2019

   Abs arXiv Video Code Poster Slides

   The goal of network representation learning is to learn low-dimensional node embeddings that capture the graph structure and are useful for solving downstream tasks. However, despite the proliferation of such methods, there is currently no study of their robustness to adversarial attacks. We provide the first adversarial vulnerability analysis on the widely used family of methods based on random walks. We derive efficient adversarial perturbations that poison the network structure and have a negative effect on both the quality of the embeddings and the downstream tasks. We further show that our attacks are transferable since they generalize to many models and are successful even when the attacker is restricted.

3. ICLR

   Predict then Propagate: Graph Neural Networks meet Personalized PageRank

   Johannes Gasteiger, Aleksandar Bojchevski, and Stephan Günnemann

   In International Conference on Learning Representations, ICLR, 2019

   Abs arXiv Code Poster

   Neural message passing algorithms for semi-supervised classification on graphs have recently achieved great success. However, for classifying a node these methods only consider nodes that are a few propagation steps away and the size of this utilized neighborhood is hard to extend. In this paper, we use the relationship between graph convolutional networks (GCN) and PageRank to derive an improved propagation scheme based on personalized PageRank. We utilize this propagation procedure to construct a simple model, personalized propagation of neural predictions (PPNP), and its fast approximation, APPNP. Our model’s training time is on par or faster and its number of parameters on par or lower than previous models. It leverages a large, adjustable neighborhood for classification and can be easily combined with any neural network. We show that this model outperforms several recently proposed methods for semi-supervised classification in the most thorough study done so far for GCN-like models. Our implementation is available online.

2018

1. ICML Oral

   NetGAN: Generating Graphs via Random Walks

   Aleksandar Bojchevski^*, Oleksandr Shchur^*, Daniel Zügner^*, and Stephan Günnemann

   In International Conference on Machine Learning, ICML, 2018

   Abs arXiv Code Poster Slides

   We propose NetGAN – the first implicit generative model for graphs able to mimic real-world networks. We pose the problem of graph generation as learning the distribution of biased random walks over the input graph. The proposed model is based on a stochastic neural network that generates discrete output samples and is trained using the Wasserstein GAN objective. NetGAN is able to produce graphs that exhibit well-known network patterns without explicitly specifying them in the model definition. At the same time, our model exhibits strong generalization properties, as highlighted by its competitive link prediction performance, despite not being trained specifically for this task. Being the first approach to combine both of these desirable properties, NetGAN opens exciting avenues for further research.

2. ICLR

   Deep Gaussian Embedding of Graphs: Unsupervised Inductive Learning via Ranking

   Aleksandar Bojchevski and Stephan Günnemann

   In International Conference on Learning Representations, ICLR, 2018

   Abs arXiv Code Poster

   Methods that learn representations of nodes in a graph play a critical role in network analysis since they enable many downstream learning tasks. We propose Graph2Gauss – an approach that can efficiently learn versatile node embeddings on large scale (attributed) graphs that show strong performance on tasks such as link prediction and node classification. Unlike most approaches that represent nodes as point vectors in a low-dimensional continuous space, we embed each node as a Gaussian distribution, allowing us to capture uncertainty about the representation. Furthermore, we propose an unsupervised method that handles inductive learning scenarios and is applicable to different types of graphs: plain/attributed, directed/undirected. By leveraging both the network structure and the associated node attributes, we are able to generalize to unseen nodes without additional training. To learn the embeddings we adopt a personalized ranking formulation w.r.t. the node distances that exploits the natural ordering of the nodes imposed by the network structure. Experiments on real world networks demonstrate the high performance of our approach, outperforming state-of-the-art network embedding methods on several different tasks. Additionally, we demonstrate the benefits of modeling uncertainty – by analyzing it we can estimate neighborhood diversity and detect the intrinsic latent dimensionality of a graph.

3. AAAI

   Bayesian Robust Attributed Graph Clustering: Joint Learning of Partial Anomalies and Group Structure

   Aleksandar Bojchevski and Stephan Günnemann

   In Conference on Artificial Intelligence, AAAI, 2018

   Abs PDF Code Poster

   We study the problem of robust attributed graph clustering. In real data, the clustering structure is often obfuscated due to anomalies or corruptions. While robust methods have been recently introduced that handle anomalies as part of the clustering process, they all fail to account for one core aspect: Since attributed graphs consist of two views (network structure and attributes) anomalies might materialize only partially, i.e. instances might be corrupted in one view but perfectly fit in the other. In this case, we can still derive meaningful cluster assignments. Existing works only consider complete anomalies. In this paper, we present a novel probabilistic generative model (PAICAN) that explicitly models partial anomalies by generalizing ideas of Degree Corrected Stochastic Block Models and Bernoulli Mixture Models. We provide a highly scalable variational inference approach with runtime complexity linear in the number of edges. The robustness of our model w.r.t. anomalies is demonstrated by our experimental study, outperforming state-of-the-art competitors.

4. BMC

   LocText: Relation Extraction of Protein Localizations to Assist Database Curation

   Juan Miguel Cejuela, Shrikant Vinchurkar, Tatyana Goldberg, Madhukar S. Prabhu Shankar, Ashish Baghudana, Aleksandar Bojchevski, Carsten Uhlig, André Ofner, Pandu Raharja-Liu, Lars Juhl Jensen, and others

   BMC Bioinformatics, 2018

   Abs HTML Code

   The subcellular localization of a protein is an important aspect of its function. However, the experimental annotation of locations is not even complete for well-studied model organisms. Text mining might aid database curators to add experimental annotations from the scientific literature. Existing extraction methods have difficulties to distinguish relationships between proteins and cellular locations co-mentioned in the same sentence. LocText was created as a new method to extract protein locations from abstracts and full texts. LocText learned patterns from syntax parse trees and was trained and evaluated on a newly improved LocTextCorpus. Combined with an automatic named-entity recognizer, LocText achieved high precision (P = 86%±4). After completing development, we mined the latest research publications for three organisms: human (Homo sapiens), budding yeast (Saccharomyces cerevisiae), and thale cress (Arabidopsis thaliana). Examining 60 novel, text-mined annotations, we found that 65% (human), 85% (yeast), and 80% (cress) were correct. Of all validated annotations, 40% were completely novel, i.e. did neither appear in the annotations nor the text descriptions of Swiss-Prot. LocText provides a cost-effective, semi-automated workflow to assist database curators in identifying novel protein localization annotations. The annotations suggested through text-mining would be verified by experts to guarantee high-quality standards of manually-curated databases such as Swiss-Prot.

2017

1. KDD

   Robust Spectral Clustering for Noisy Data: Modeling Sparse Corruptions Improves Latent Embeddings

   Aleksandar Bojchevski, Yves Matkovic, and Stephan Günnemann

   In International Conference on Knowledge Discovery and Data Mining, KDD, 2017

   Abs HTML PDF Code Poster

   Spectral clustering is one of the most prominent clustering approaches. However, it is highly sensitive to noisy input data. In this work, we propose a robust spectral clustering technique able to handle such scenarios. To achieve this goal, we propose a sparse and latent decomposition of the similarity graph used in spectral clustering. In our model, we jointly learn the spectral embedding as well as the corrupted data – thus, enhancing the clustering performance overall. We propose algorithmic solutions to all three established variants of spectral clustering, each showing linear complexity in the number of edges. Our experimental analysis confirms the significant potential of our approach for robust spectral clustering. Supplementary material is available at www.kdd.in.tum.de/RSC.

2. BioInf

   nala: Text Mining Natural Language Mutation Mentions

   Juan Miguel Cejuela, Aleksandar Bojchevski, Carsten Uhlig, Rustem Bekmukhametov, Sanjeev Kumar Karn, Shpend Mahmuti, Ashish Baghudana, Ankit Dubey, Venkata P Satagopam, and Burkhard Rost

   Bioinformatics, 2017

   Abs HTML Code

   The extraction of sequence variants from the literature remains an important task. Existing methods primarily target standard (ST) mutation mentions (e.g. ‘E6V’), leaving relevant mentions natural language (NL) largely untapped (e.g. ‘glutamic acid was substituted by valine at residue 6’). We introduced three new corpora suggesting named-entity recognition (NER) to be more challenging than anticipated: 28–77% of all articles contained mentions only available in NL. Our new method nala captured NL and ST by combining conditional random fields with word embedding features learned unsupervised from the entire PubMed. In our hands, nala substantially outperformed the state-of-the-art. For instance, we compared all unique mentions in new discoveries correctly detected by any of three methods (SETH, tmVar, or nala). Neither SETH nor tmVar discovered anything missed by nala, while nala uniquely tagged 33% mentions. For NL mentions the corresponding value shot up to 100% nala-only.

workshops

1. CAO Oral, Best Poster

   CATS: Conformalized Adaptive Test-Time Scaling

   Sadegh Mohammad Akhondzadeh, Soroush H. Zargarbashi, Simone Antonelli, and Aleksandar Bojchevski

   In Catch, Adapt and Operate Workshop at ICLR, 2026

   Abs HTML PDF

   Reasoning language models (RLMs) have shown significant improvements through test-time scaling. However, these gains come at the cost of more resource-intensive and slower inference, leading to latencies that directly impacts user experience. This cost can be reduced by avoiding overthinking and responding quickly to simple queries. Ideally, models should adapt their thinking strategy to the difficulty of the task. Such adaptivity is increasingly feasible because recent foundation models already provide discrete options for reasoning effort (e.g., low, high). We provide a framework for adaptive test-time scaling with a guarantee of controlled risk. Our method estimates task difficulty based on the probability of success at each reasoning level (while keeping additional cost minimal) and adjusts its output via conformal risk control. Specifically, our method guarantees that the probability of producing an incorrect answer remains below a user-specified tolerance level, while adaptively allocating less compute to easier queries and more to harder ones. Our results demonstrate that an appropriate reasoning level can be selected automatically while ensuring rigorous statistical guarantees.

2. PDTAI Oral

   Test-Time Training Undermines Safety Guardrails

   Simone Antonelli^*, Sadegh Mohammad Akhondzadeh^*, and Aleksandar Bojchevski

   In Principled Design for Trustworthy AI Workshop at ICLR, 2026

   Abs arXiv

   Test-Time Training (TTT) is an emerging paradigm that enables models to adapt their parameters during inference, improving performance on tasks such as few-shot learning, retrieval-augmented generation, and complex reasoning. However, this dynamic adaptation introduces new vulnerabilities that adversaries can exploit to jailbreak models. We identify three threat models for TTT and demonstrate how attackers can leverage them to bypass safety filters. Our results show that TTT can significantly increase the Attack Success Rate (ASR) and the ASR over 10 generation trials (ASR@10). For example, under LoRA, the few-shot and generation-phase threat models achieve an average ASR@10 of 95% and 93% respectively, across models from different families and scales. These vulnerabilities transfer to production fine-tuning APIs. We also show that TTT-induced overfitting can produce degenerate outputs that inflate ASR under standard judges, and propose a validity-aware evaluation to correct for this. Our findings suggest that TTT exposes a new attack surface, strengthens attacks, and undermines existing safety guardrails. As a first step toward defense, we propose a lightweight provider-side detector that flags TTT requests via the perplexity shift on a private harmful holdout, but robust deployment will ultimately require dynamic alignment.

3. PLDC

   Pitfalls in Evaluating GNNs under Label Poisoning Attacks

   Vijay Lingam, Sadegh Mohammad Akhondzadeh, and Aleksandar Bojchevski

   In Workshop on Pitfalls of Limited Data and Computation for Trustworthy ML at ICLR, 2023

   Abs HTML PDF

   Graph Neural Networks (GNNs) have shown impressive performance on several graph-based tasks. However, recent research on adversarial attacks shows how sensitive GNNs are to node/edge/label perturbations. Of particular interest is the label poisoning attack, where flipping an unnoticeable fraction of training labels can adversely affect GNNs’ performance. While several such attacks were proposed, the latent flaws in the evaluation setup cloud the true effectiveness of the attacks. In this work, we uncover 5 frequent pitfalls in the evaluation setup that plague all existing label-poisoning attacks for GNNs. We observe for some settings that the state-of-the-art attacks are no better than a random label-flipping attack. We propose and advocate for a new evaluation setup that remedies the shortcomings, and can help gauge the potency of label-poisoning attacks fairly. Post remedying the pitfalls, on the Cora-ML dataset, we see a difference in performance of up to 19.37%.

4. DLG

   Attacking Graph Neural Networks at Scale

   Simon Geisler, Daniel Zügner, Aleksandar Bojchevski, and Stephan Günnemann

   In Deep Learning on Graphs Workshop at AAAI, 2021

   HTML
5. MLG

   Is PageRank All You Need for Scalable Graph Neural Networks?

   Aleksandar Bojchevski, Johannes Gasteiger, Bryan Perozzi, Martin Blais, Amol Kapoor, Michal Lukasik, and Stephan Günnemann

   In International Workshop on Mining and Learning with Graphs, MLG, 2019

   Abs PDF Code

   Graph neural networks (GNNs) have emerged as a powerful approach for solving many network mining tasks. However, efficiently utilizing them on web-scale data remains a challenge despite related advances in research. Most recently proposed scalable GNNs rely on an expensive recursive message-passing procedure to propagate information through the graph. We circumvent this limitation by leveraging connections between GNNs and personalized PageRank and we develop a model that incorporates multi-hop neighborhood information in a single (non-recursive) step. Our work-in-progress approach PPRGo is significantly faster than multi-hop models while maintaining state-of-the-art prediction performance. We demonstrate the strengths and scalability of our approach on graphs orders of magnitude larger than typically considered in the literature.

6. GEM

   Dual-primal Graph Convolutional Networks

   Federico Monti, Oleksandr Shchur, Aleksandar Bojchevski, Or Litany, Stephan Günnemann, and Michael M Bronstein

   In Graph Embedding and Mining Workshop, GEM, 2019

   Abs arXiv

   In recent years, there has been a surge of interest in developing deep learning methods for non-Euclidean structured data such as graphs. In this paper, we propose Dual-Primal Graph CNN, a graph convolutional architecture that alternates convolution-like operations on the graph and its dual. Our approach allows to learn both vertex- and edge features and generalizes the previous graph attention (GAT) model. We provide extensive experimental validation showing state-of-the-art results on a variety of tasks tested on established graph benchmarks, including CORA and Citeseer citation networks as well as MovieLens, Flixter, Douban and Yahoo Music graph-guided recommender systems.

7. R2L

   Pitfalls of Graph Neural Network Evaluation

   Oleksandr Shchur, Maximilian Mumme, Aleksandar Bojchevski, and Stephan Günnemann

   In Relational Representation Learning Workshop, R2L, 2018

   Abs arXiv Code

   Semi-supervised node classification in graphs is a fundamental problem in graph mining, and the recently proposed graph neural networks (GNNs) have achieved unparalleled results on this task. Due to their massive success, GNNs have attracted a lot of attention, and many novel architectures have been put forward. In this paper we show that existing evaluation strategies for GNN models have serious shortcomings. We show that using the same train/validation/test splits of the same datasets, as well as making significant changes to the training procedure (e.g. early stopping criteria) precludes a fair comparison of different architectures. We perform a thorough empirical evaluation of four prominent GNN models and show that considering different splits of the data leads to dramatically different rankings of models. Even more importantly, our findings suggest that simpler GNN architectures are able to outperform the more sophisticated ones if the hyperparameters and the training procedure are tuned fairly for all models.

8. ICDMW

   Anomaly Detection in Car-Booking Graphs

   Oleksandr Shchur, Aleksandar Bojchevski, Mohamed Farghal, Stephan Günnemann, and Yusuf Saber

   In International Conference on Data Mining Workshops, ICDM, 2018

   Abs HTML

   The use of car-booking services has gained massive popularity in the recent years – which led to an increasing number of fraudsters that try to game these systems. In this paper we describe a framework for fraud detection in car-booking systems. Our core idea lies in casting this problem as an instance of anomaly detection in temporal graphs. Specifically, we use unsupervised techniques, such as dense subblock discovery, to detect suspicious activity. The proposed framework is able to adapt to the variations in the data inherent to the car-booking setting, and detects fraud with high precision. This work is performed in collaboration with Careem, where the described framework is currently being deployed in production.

theses

1. PhD

   Machine Learning on Graphs in the Presence of Noise and Adversaries

   Aleksandar Bojchevski

   Technical University of Munich, 2020

   Abs

   From protein interactions to social networks, complex systems of interlinked entities are endemic in a connected world and graphs are a powerful abstraction for capturing their structure. Accordingly, we have a rich literature of machine learning techniques for graph data to solve problems ranging from fraud detection to cancer classification. Since in reality data is unreliable understanding the robustness of these techniques to noise and adversaries is critical. The contributions of this thesis deepen our understanding of robustness for three types of models: unsupervised, generative, and semi-supervised. First, we derive a noise-resilient variant of the classical spectral embedding, and we introduce Gaussian embeddings that represent nodes as distributions to capture uncertainty. Then we study the sensitivity of node embeddings to graph poisoning. Next, we develop a generative model that explicitly accounts for anomalies and detects clusters obfuscated by noise. Finally, we derive provable robustness guarantees. We propose the first certificate w.r.t. structure perturbations for a large class of PageRank-based models, and we derive a general certificate for discrete data applicable to any graph classifier.

2. MSc

   Semi-supervised Learning for Biomedical Named-Entity Recognition

   Aleksandar Bojchevski

   Technical University of Munich, 2015

   Abs PDF Code Slides

   As the volume of published research in the biomedical domain increases, the need for effective information extraction systems grows in parallel. In this context, the task of named-entity recognition (NER) is essential. NER is defined as the classification of words in free text that represent predefined categories such as genes, proteins or other entities. As a specific application of NER, the main focus of this thesis is the recognition of mutation mentions from the biomedical literature. More specifically we aim to create a model able to recognize mutation mentions expressed in natural language. The current state-of-the-art method, tmVar is only able to recognize a small subset of standard or semi-standard mentions. Our method both outperforms tmVar on those types of mentions and is also able to recognize natural language (NL) mentions. Previously no other method considered NL mutation mentions. The performance of NER machine learning models is intrinsically limited by the availability of high-quality-annotated corpora. The construction of such corpora is costly – specially when expert annotators are required. In the biomedical domain, the difficulty of the task is even greater, since the number of possible named entities is higher and keeps growing with new discoveries. To combat the lack of large annotated corpora, we turn to the exploitation of large volumes of unlabeled text, applying a semi-supervised learning approach. Using techniques for unsupervised feature learning we aim to increase the performance of traditional NER models. More specifically, this thesis focuses on augmenting common conditional random field (CRF) approaches combined with novel word representation features learned from large bodies of biomedical text. Furthermore, using an active learning approach we extend an existing corpus of mutation mentions (IDP4) with additional NL mentions. Finally, and in support of evaluating our semi-supervised learning approach, we develop a complete pipeline for biomedical named-entity recognition including preprocessing steps, feature generation, model learning and normalized predictions. Our extended corpus, NER tool and pipeline framework are all open sourced on GitHub.

3. BEng

   Personality Prediction Based on Information from Social Networks

   Aleksandar Bojchevski

   Ss. Cyril and Methodius University, 2013